In heat pump applications, the switchover from the heating to the cooling mode, and vice versa, reverses the direction of flow for the refrigerant such that the coils serving as the condenser and evaporator, respectively, reverse functions. Where the compressor operates in a single direction, the change in the direction of the flow is generally achieved through a valving arrangement located externally of the compressor. If the compressor itself is reversible, it can be selectively run in either direction to, thereby, achieve the desired direction of flow. The simple reversal of the motor and, thereby, the compressor is not, in and of itself, sufficient to produce a compressor with satisfactory performance in both directions. This unequal performance in both directions is due to the switching between high and low side compressor operation, the change in the cooling requirements and the cooling flow, flow volumes, the reversal of porting function and direction of opening/closing, etc.
In a fixed vane or rolling piston type of compressor, a cylindrical rolling piston is in linear rolling contact with the cylindrical wall of the piston chamber. The rolling piston is moved by an eccentric located on the crankshaft and has a rolling contact with the wall of the piston chamber and defines therewith a crescent shaped chamber extending for almost 360.degree.. A vane is radially movable and engages the rolling piston so as to divide the crescent shaped chamber into a suction chamber and a discharge chamber with their relative instantaneous volumes depending upon the location of the linear contact between the rolling piston and the wall of the piston chamber.